1. Field of the Invention
The present invention relates to a heat-sensitive hot-melt image transfer sheet for use in a thermal printer.
2. Prior Art
There are known various heat-sensitive hot-melt image transfer sheets for use in thermal printers. One conventional heat-sensitive hot-melt image transfer sheet comprises a base of polyester having a thickness ranging from 3 .mu.m to 9 .mu.m and a layer of hot-melt ink coated uniformly on one surface of the base. In use, the heat-sensitive hot-melt image transfer sheet is set in a thermal printer while the hot-melt ink layer is being held in contact with an image recording sheet and the reverse side of the image transfer sheet is in contact with the thermal head of the printer. Then, an electric current representative of an image to be transferred is supplied to the thermal head to heat the same. The hot-melt ink is thus melted in a selected pattern such as characters or a graphic figure by the heat of the thermal head, and the melted ink pattern is transferred to the image recording sheet.
When a colored image is to be recorded on a recording sheet by a thermal printer, there is employed a heat-sensitive hot-melt image transfer sheet which comprises a base coated with full-page patterns of hot-melt inks of different colors such as yellow, magenta, cyan, and black. The hot-melt inks are successively melted by the thermal head in selected image patterns which are transferred overlappingly onto the recording sheet, thus producing a colored image thereon.
The electric current or currents supplied to the thermal head to record a desired monochromatic or colored image are varied to vary the amount of the melted ink or the amounts of melted inks, so that the area or areas in which the hot-melt ink or inks are transferred to the recording sheet are controlled to achieve a desired gradation of density of the recorded image.
If the density of a portion of an image to be recorded is low, then the level of the electric current fed to the thermal head to reproduce the density is also low. Therefore, the heat applied by the thermal head to the hot-melt ink corresponding to that image portion is absorbed by the surrounding hot-met ink, reducing the region in which the hot-melt ink is actually melted, so that the hot-melt ink will not be sufficiently transferred from the base of the image transfer sheet. As a result, dots formed on the recording sheet by the transferred hot-melt ink may lack portions thereof or become irregular in shape. When the hot-melt ink is heated to a lower temperature, its viscosity remains higher, making it difficult for an intended quantity of hot-melt ink to be separated from the image transfer sheet base.
Conversely, if the density of an image portion to be recorded is high, then the level of the supplied electric current to accomplish the density is also high. The hot-melt ink may then be melted and transferred excessively, with the result that adjacent ink dots on the recording sheet may be joined to or combined with each other. When the hot-melt ink is heated to a higher temperature, its viscosity becomes lower, with the result that surrounding areas of hot-melt ink as well an intended area thereof will be separated from the image transfer sheet base. Thus, an excessive amount of hot-melt ink tends to be supplied from the image transfer sheet.
When the hot-melt ink or inks are hot melted and transferred in a desired amount or amounts, the area or areas on the recording medium in which the melted ink or inks are present tend to vary from a desired size or sizes. Then, the gradations of density of the image to be recorded cannot accurately be controlled, the image cannot sharply be recorded, and, if the image is colored, the desired colors cannot be reproduced exactly.
The ink or inks of the conventional heat-sensitive hot-melt image transfer sheets are coated uniformly on the entire surface of their bases. A special ink coat applicator has heretofore been required in the production of the heat-sensitive hot-melt image transfer sheets. It has been difficult and costly to manufacture the heat-sensitive hot-melt image transfer sheets.